are the heads your wanting to use have sockets that are too small or too large for the neck ball?
if they are too small:
take a dremel and lightly reem it out and test fit it as you go until you get a good fit
if they are too large:
fill the socket full of hot glue and before it cools hold the ball up into the glue filled socket, let the glue cool, then trim off the excess. If you do it right then the glue will fill in the extra space in the socket and make a new tight ball socket for the smaller ball...